846 



SCIENCE. 



[N. S. Vol. VII. No. 182. 



Science, i. e., by the mechanical action of 

 frost on sandstones exhibiting a tendency 

 to conchoidal fracture. 



These natural bridges occur in the strip 

 of very rough country formed by the out- 

 crop of the Basal Coal Measure Conglom- 

 erate, where it is underlaid first by shale 

 and then by lower Carboniferous limestone 

 and shaly sandstone. This strip is a part 

 of a deeply multi- dissected plateau, known 

 farther south as the Cumberland Plateau. 

 The tributary streams, deeply buried below 

 the old plateau level, have along the mar- 

 gins of the strip cut down into the underly- 

 ing shale, limestone and shaly sandstone. 

 They have extended their ramifications up 

 the steep slopes to the base of the Conglom- 

 erate, where they have hollowed out their 

 virtual sources in the shale, undermined 

 the Conglomerate cliflfs and thus formed vast 

 amphitheaters, or ' rock-houses.' The ex- 

 tent of these is often seemingly out of all 

 proportion to the size of the stream leading 

 away from them. These streams some- 

 times find continuation above the cliffs as 

 wet- weather streamlets and during the 

 times of their flow plunge over the escarp- 

 ments in picturesque waterfalls. More 

 commonly, however, there is no gathering 

 ground above. By the recession of sources 

 the watersheds have been reduced to the 

 narrowest ridges, which are often quite in- 

 accessible. In a number of instances two 

 streams in their recession have met in the 

 shales under the Conglomerate. Two ' rock- 

 houses ' have met back to back and formed 

 a ' Natural Bridge.' There are three of 

 these bridges in Kentucky that are begin- 

 ning to have something more than a local 

 celebrity. One of these is in Pulaski, one 

 in Wolfe and one in Powell county. Views 

 of the Powell county bridge accompany this 

 sketch. Perhaps a tendency to conchoidal 

 fracture in the coarse sandstone has favored 

 the hollowing- out process that has resulted 

 in these natural arches, but in the main they 



are due to the mechanical action of water 

 causing a recession of sources in the way 

 above indicated. 



Akthur M. Millek. 

 State College, Lexington", Ky. 



THE INTERNATIONAL AERONAUTICAL CON- 

 FERENCE AT STEASSBURG. 



The meeting, ten weeks ago, of the In- 

 ternational Aeronautical Committee ap- 

 pointed by the Paris Meteorological Confer- 

 ence of 1896 was noteworthy in two re- 

 spects. First, it marked the beginning of 

 a new era in meteorological investigation,, 

 as shown by an organized effort to cut loose 

 from observatories on the earth and to 

 study the conditions of the free air ; and, 

 second, by the assembling at Strassburg of 

 French and Germans, political question* 

 were held to be subservient to the con- 

 quest of the high atmosphere and the ex- 

 tension of the common realm of science. 

 OflScial and private hospitality was abun- 

 dant and the utmost good fellowship pre- 

 vailed among the members of the Com- 

 mittee and the other meteorologists,^ 

 physicists and aeronauts who were present 

 by invitation. It was regretted, although 

 hardly a surprise, that there was no one 

 from Great Britain, where, since Glaisher's- 

 remarkable balloon ascension, little has- 

 been done to explore the free air. The fol- 

 lowing named members of the Committee 

 were in attendance : Professor Hergesell, 

 the President, of Strassburg ; MM. de Fon- 

 vielle, the Secretary, Cailletet and Besangon,. 

 of Paris ; Drs. Assmann and Person, of Ber- 

 lin ; Professor Erk, of Munich ; General- 

 Major Ry.kateheff and Colonel Kowanko, 

 of St. Petersburg ; Mr. Rotch, of Boston, 

 United States. 



The methods discussed' for obtaining ob- 

 servations in the free air were balloons 

 with aeronauts ; ballons- sondes, or unmanned 

 balloons to carry self-recording instruments 

 to the height of ten miles or more ; and. 



